The present invention relates to a skidding control method and device for a four wheel drive power transmission system, and more particularly relates to such a skidding control method and device for such a four wheel drive power transmission system for a vehicle such as an automobile adapted for four wheel drive operation, particularly adapted to control a differential device which is provided for distributing power between the front wheels of the vehicle and the rear wheels of the vehicle, in which the construction and operation thereof are improved so as to improve the quality of anti skidding control and thereby improve vehicle drivability.
The present invention has been described in Japanese Patent Applications Ser. Nos. 60-228073 and 60-228074 (1985), filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the text of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
Nowadays a greatly increasing number of automotive vehicles are being constructed with four wheel drive transmission systems, because such four wheel drive operation, in which all four wheels of the vehicle are powered from its engine via its transmission, is very suitable for driving on poor or slippery road surfaces such as in mud or over bad ground, or upon roads covered with mud, snow, ice, or rain. In other words, four wheel drive operation provides a much higher degree of stability and drivability for the vehicle in conditions where the coefficient of friction between the wheels and the surface upon which the vehicle is riding is relatively low. Also, four wheel drive operation is beneficial for aiding with hill climbing characteristics and high speed stability characteristics.
In a four wheel drive transmission system for such an automotive vehicle, it is usual to provide a differential device for distributing rotational power between the front wheels of the vehicle and the rear wheels of the vehicle, as well as the per se conventional rear differential device that provides differential action between the two rear vehicle wheels and the also per se conventional front differential device that provides differential action between the two front vehicle wheels. Such a front-rear differential device is provided in order to provide a differential action between said front vehicle wheels (considered as a pair) and said rear vehicle wheels (also considered as a pair) when the vehicle is turning around a curve. However, a problem that arises with the provision of such a front-rear differential device is that, if one of the four vehicle wheels should break away from the road surface and should start to spin, then no drive is provided to the other three vehicle wheels.
In order to counter this effect, it has been practiced to provide a device to such a front-rear differential device which prevents said front-rear differential device from performing differential action, in a selective fashion. When such a differential action inhibition means is actuated, it causes the differential action provided by said front-rear differential device between the front vehicle wheels and the rear vehicle wheels to be prevented, and instead said front vehicle wheels, considered as a pair, are driven from the vehicle engine, and also said rear vehicle wheels, considered as a pair, are independently driven from said engine. Thereby, the problem outlined above, of loss of power to the other vehicle wheels when one of the vehicle wheels starts to spin, is obviated.
Further, when driving upon a road surface with a low coefficient of friction in such a four wheel drive vehicle, the effect of such a front-rear differential device is that the various vehicle wheels can skid independently. In order to counter this, in Japanese Utility Model Application Laying Open Publication Ser. No. 59-155225 (1984), which it is not intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, there is disclosed the concept of, when the coefficient of friction between the steered wheels (the front wheels) and the road surface is less than a determinate value, and also the rear differential itself is not locked, displaying an indication instructing the vehicle driver to lock the front-rear differential device so as to suppress its differential effect.
However, the following problems arise with such a concept.
First, when driving upon a road surface with a low coefficient of friction in such a four wheel drive vehicle, even though an indication instructing the vehicle driver to lock the front-rear differential device so as to suppress its differential effect may be displayed, the actual locking of said front-rear differential device must be manually performed by the vehicle driver according to his or her own judgement and decision. Unfortunately, it is precisely upon this sort of road surface that the attention of the vehicle driver is preempted by more pressing problems of vehicle control, since the driving and braking stability of the vehicle are deteriorated according to reduction of the road surface coefficient of friction and accordingly operation of the vehicle is more difficult, and thus the added burden on said driver of monitoring and responding to such a differential device locking indication is unrealistic and unreasonable.
Second, since in such a proposal as outlined above only the coefficient of friction between the steered vehicle wheels, i.e. the front wheels, and the road surface is monitored, in the event that it should be the rear wheels of the vehicle which skid first, no proper locking ordering for the front-rear differential device is performed.
Another important consideration in this connection is that it is important to release the locking of the front-rear differential device as soon as appropriate, because, if said front-rear differential device is maintained as locked over an extensive time period, then the inevitable differences in the rotational speeds between the front wheels and the rear wheels are not absorbed, and undue tire wear and transmission wear are caused, and further the vehicle performance and fuel economy are deteriorated. On the other hand, said locking of the front-rear differential device should of course not be released before such release is in fact appropriate, i.e. if skidding of any vehicle wheel is continuing.